Butterflies Like Mates With a Twist

Alison Sweeney, a biologist at Duke University, was studying deep-forest butterflies in South America when she realized that color alone is not the key to their beauty. To other butterflies, iridescencethe way light rays align when they reflect off the wingis at least as fetching as the fancy colors. Sweeney knew that butterflies have an excellent ability to detect polarization, the direction in which the waves in a beam of light are oscillating. Then she discovered that iridescent markings on some butterfly wings create polarized patterns. "It struck me as odd that with all the colors, they would have taken the energy to evolve iridescence as well, so I assumed it must have a significant purpose," Sweeney says.

Heliconius cydno (top) lures lovers with polarized patterns, while H. melpomene malleti (bottom) uses plain old color. The right halves of the wings are seen here through a polarizing filter.Photographs courtesy of Alison Sweeney.

Sweeney gathered a group of male Heliconius butterflies, a highly polarized genus, and showed them two sets of wings from Heliconius females. One set was displayed normally; the other was shown through a filter that blotted out the polarization effects. The males swarmed toward the normal wings and largely ignored the filtered ones. When the insects could not see the polarization patterns, they were not interested. Sweeney claims this is the first known instance of a terrestrial species using polarized light for anything other than navigation. "I wouldn't be surprised if there were other insect species using this type of thing for communication," she says. "It's just hard to explore what you can't really see."